a) Field of the Invention
This invention relates to a method of curing anaerobic compositions through thick bondlines, and to compositions for use in the method. The invention is particularly concerned with anaerobic adhesives which are used in engineering applications such as gasketting, retaining, sealing and thread-locking.
b) Description of the Related Art
Redox initiated room temperature bulk polymerisation of monomeric and telechelic type (meth)acrylates is generally triggered by metallic species inherent on machinery parts. The "cure engine" which supplies a radical flux on demand may comprise a hydroperoxide together with common accelerators. (STAMPER, D. J., Curing characteristics of anaerobic sealants and adhesives. Brit. Polym. J., 15 (1983) 34-9).
The decomposition of hydroperoxide in the presence of amine is well known to be catalysed by transition metals, particularly cupric or ferrous species for example. The appropriate metallic species are provided by substrate surfaces or alternatively, by specific activating treatment when substrates lack activity. An overview of anaerobic adhesives has been recently published by Boeder. (BOEDER, C. W., "Anaerobic and Structural Acrylic Adhesives" in "Structural Adhesives - Chemistry and Technology", ed. S. R. Hartshorn, Plenum Press, New York 1986, p.217).
Surface initiated redox polymerisation is reasonably efficient and products may be developed with a range of characteristics tailored to meet end user requirements. Specific requirements might, for example, relate to cure speed or fixture time as such parameters may be of extreme importance in on-line manufacturing processes which involve bonding stages. Cure speed will, to a degree, correlate with extent of cure through a given bondline thickness. Thus, cure through gap or cure through volume (CTV), is a further feature of anaerobic adhesives which may require characterisation.
In the majority of their end usage, anaerobic adhesives are employed in so-called "zero gap" situations, i.e. when substrates to be bonded are in intimate contact (in reality zero gap corresponds to some 5-15 .mu.m). Under such circumstances bonded assemblies will have high strength and performance as a consequence of an essentially homogeneous cure initiated from both sides of a thin (c. 10 .mu.m) bondline. In specialised applications, however, the substrates to be bonded may, by intention or otherwise, have non-uniform surface profiles and excursions of several hundred micrometers may be present between them. In such circumstances, bulk adhesive in the centre of a thick bondline (hundreds of micrometers) is distanced from the surface source of catalytic metallic species. Furthermore, the adhesive layers closest to the substrate surface will already be experiencing cure so that the prospect for metal ion diffusion into the bulk is reduced due to the physical barrier to transport established by crosslinked polymer network formation at the substrate-adhesive interface. The heterogeneous cure in this latter case will lead to joints with performance and strengths dependent upon the relative fractional area between substrates which is set at large gap when conventional anaerobic adhesives are employed. (McArdle et. al., Plastics, Rubber and Composites Processing and Applications 16 (1991) 245-253).
Otsu et. al. (j. Polym. Sci (A-1) Z, 3329, 1969) teach that Dimethyl benzyl aniline chloride (DMBAC), a quaternary ammonium salt, is an efficient component of a redox couple for radical initiation of acrylic polymerisations in solvent. This salt is apparently more efficient in this regard than its parent amine, N,N-dimethyl aniline. However Otsu et. al. do not provide any teaching about anaerobic compositions which use saccharin as an accelerator. Further reference has been made to the participation of quaternary ammonium salt in vinyl polymerisation in solution by Ghosh et al. (Eur. Polymer J, 14, 855, 1978) and Rasmussen et. al. (Makromol. Chem, 182, 701, 1981). In the former study in both dilute and concentrated solution, mechanisms have been proposed wherein cetyl trimethyl ammonium bromide interacts with benzoyl peroxide to produce radicals capable of initiating polymerisations. In the latter study the quaternary (quat) salt was merely used as an agent to solubilise other well known initiators, e.g. peroxydisulphate. Still further studies in emulsion systems teach the participation of quat salts in bulk polymerisation of methyl methacrylate (Polym. Bull., 21, 151, 1989). The trioctyl methyl ammonium salts [(TOMAX) where X=anion] that were used were sufficiently soluble in the bulk state and again a mechanism has been proposed. That quat salts are active in the decomposition of hydroperoxides as well as peroxides is further emphasised by the study of Napadensty et. al. (J. Chem. Soc. Chem. Commun. 65, 1991) in which the catalytic effect of the quat salt in the decomposition of tetralin hydroperoxide has been studied. In this study, the catalytic activity of the quat salt decreases as the anion is changed from chloride through bromide to iodide.
Czechoslovak Patents CS 169915-B and CS 169916-B (1977) describe anaerobic compositions which contain certain quaternary ammonium salts formed between heterocyclic bases and (meth)acrylic acid. However these compositions do not contain saccharin as an accelerator.
Quaternary salts have not been previously employed as part of a cure system for the purpose of enhancing cure through thick bondlines. Whilst very active anaerobic formulations can be made with traditional anaerobic accelerators, e.g. those of Table 4 on page 227-8 of the book "Structural Adhesives - Chemistry and Technology" edited by S. R. Hartshorn published by Plenum Press (1986), these do not demonstrate good cure through volume (CTV). If a cure system is adjusted, CTV may be achieved but at the expense of stability of the formulation so that its commercial value is worthless.